Protective circuits



fi- 1957 c. E. s. RIDGERS 2,802,151

PROTECTIVE cmcuns Filed'Ndv. '7, 1955 RLB2 RLA2 Min/mg C, E. 5. RIDGE/Q5 United States Patent signor to The Decca Record Company Limited, London, England, a British company Application November 7, 1955, Serial No. 545,499

Claims priority, application Great Britain March 30, 1955 8 Claims. (Cl. 317-40) This invention relates to circuits for protecting equipment embodying an electronic valve or valves in the event of a flash-over occurring in a valve such as to cause a heavy current to be drawn from the supply source.

In some equipment it is necessary to apply pulses of high voltage to a valve and flash-over between electrodes of the valve is not an uncommon fault in such equipmerit. For example, in' microwave radar apparatus operating on the highest frequencies, it may be necessary to use hard valve modulators for modulating magnetron oscillators because the pulse durations must be very short. In such equipment it is necessary to protect the apparatus from the effect of flash-over in the modulator valve. It is one object of the present invention to provide improved protective equipment for such purposes.

According to this invention a circuit for protecting equipment embodying an electronic valve in the event of an excessive current being drawn by the valve comprises a supply source for supplying two alternative voltages one higher than the other, relay operated switch means for applying the two alternative voltages to said equipment in two stages, the lower voltage being applied before the big-her voltage, and means responsive to the current drawn from the supply source arranged immediately to interrupt the high voltage supply in the event of an excessively large current being drawn and subsequently to initiate operation of the switch means for applying the high voltage in two stages. With this arrangement, if a flash-overshould occur which causes an excessive current to be drawn, the high voltage will be switched off from the valve and subsequently the reduced voltage will be applied to the valve. If the apparatus is now functioning correctly, the full voltage will then be applied to the valve and the apparatus will be back in normal: operation. If, however, the flash-over still occurs when the reduced voltage is applied, the high voltage will again be cut oti and the full voltage will not be applied tothe circuit.

The interruption of the high voltage supply may conveniently be effected by interrupting an alternating supply circuit from which the high voltage is derived. With this arrangement, the two stages of high voltage may conveniently be obtained either by switching a tapped connectionof a transformer in the alternating supply circuit or by switching a series impedance in the alternating supply circuit.

' In one arrangement, the interruption of the high volt age supply is effected by an overload relay, the operating coil of which is arranged to be energised directly or indirectly when an excessive current is drawn, which over load relay has a first normally closed contact in the alternating supply circuit, which contact opens when the excessive current is drawn, said first normally closed contact being shunted by a normally closed contact of a delay relay, the operation of which is initiated by a second normally open contact of the overload relay, this second contact closing when the overload relay operates to complete a circuit for en'erg'ising the delay relay. The

2,802,151 Patented Aug. 6, 1957 delay relay is arranged to release after a short time interval, for example, by connecting the relay in series with a resistor and by connecting a condenser in shunt across the relay. The delay relay is provided with a second contact which during the interval that the delay relay is energised controls through a further relay the necessary switching to reduce the high voltage applied to the valves.

In the following description reference will be made to the accompanying drawings in which Figure l is a circuit diagram of one form of circuit for protecting equipment embodying an electronic valve or valves in the event of excessive current being drawn by a valve, and

Figure 2 is a diagram illustrating a modification of the circuit arrangement of Figure 1.

Referring to Figure l, the circuit to be protected is energised from the secondary winding 10 of a transformer 11, the primary winding 12 of which is connected to a source of alternating supply through a circuit including a change-over contact RLC1 which switches a tap on the winding 12 whereby the high voltage applied to the apparatus may be reduced. The circuit for the winding 12 also includes a winding 13 on a transformer 14 which, when a large current is drawn by the Winding 10, energises an overload relay RLD through a rectifier 15 in a circuit from a secondary winding 16 on the transforrner 14. In series also with the supply to the Wind'- ing 12 is a normally-closed contact RLD1 of the relay RLD. This contact RLD1 being shunted by a normallyclosed contact RLAl of a delay relay RLA.

The aforementioned change-over contact RLCI is normally in the position where reduced voltage is applied to the apparatus and this contact is controlled by relay RLC which is energised in a 50 volt direct current circuit through a second normally-closed contact RLA2 of relay RLA and a normally-open contact RLB2 of a start relay RLB. The operating coil of relay RLB is connected in series with a thermistor 18 across the 50' volt supply. The operating coil RLB is normally shunted through a bleed resistor 19 by means of the change-over contact RLBl, which contact is arranged, when the relay RLB operates, to remove the shunt across coil RLB and to put a short circuit across the thermistor 18. The relay RLA is shunted by a condenser 20 and is connected in series with a resistor 21 and with a normally open contact RLD2 of the overload relay RLD.

The operation of the above-described circuit is as fol-' lows: When the apparatus isfirst to be switched on the volt alternating current supply is applied to the input terminals 22, 23 and the 50 volt direct current supply (which may be derived by rectification of the alternating supply) is applied to terminals 22, 24. The 80 volt supply is applied via contact RLCI to transformer 11 so that a reduced voltage output is provided by the output winding 10. The 50 volt supply will gradually warm up the thermistor 18 and after a delay time which, in a typical case, might be one and a half minutes, the relay RLB operates. Contact RLBl then removes the shunt resistor 19 and short circuits the thermistor 18 to ensure that the relay operates quickly. Short-circuiting the thermistor ensures that the thermistor becomes cold ready for the next operation. The contact RLB2 completes the circuit for relay RLC through contact RLAZ. When relay RLC is energised the change-over contact RLCl operates to provide the full output voltage on the winding 10.

Suppose now that a flash-over takes place in the apparatus such that a large current is drawn through the winding 13. The relay RLD will be energised from the winding 16 and contact RLD1 will open. The transformer winding 12 is still energised through contact RLAI but the closing also of contact RLD2 operates relay RLA-. This in turn opens contact RLAl so cutting off the supply to the winding 12. The operation of relay RLA also opens contact RLAZ thereby releasing relay RLC and changing over the contact RLCl to the reduced voltage position. When the supply to winding 12 has been interrupted, .the relay RLD will release so closing contact RLDl and thus restoring the supply to winding 12 so that the reduced voltage is now supplied by winding 10. The release of relay RLD will open contact RLDZ but the condenser has by now been charged and this charge will maintain the relay RLA in the operated condition for a short time. After the relay RLA has released, the contact RLAl will close in shunt across the already closed contact RLDI. The contact RLA2 will also close to complete the circuit for relay RLC and so change over the contact RLCl to apply full voltage to the apparatus. In a typical case the total time from the initial operation of the overload relay RLD on the occurrence of a hash over until the restoration of full voltage by the operation of relay RLC may be of the order of l to 2 seconds.

In Figure 2, there is illustrated a modification of part of the circuit arrangement of Figure 1 in which the relay contact RLCl is adapted to switch, between two alternative paths, the connection from the input terminal 23 to a tap on the transformer winding 12. These two paths include resistors 31, 32 of different value, the resistor 31 being of higher value than the resistor 32 so that in the normal position of the contact RLC a lower voltage is applied to the winding 12 than is applied when the contact is in the operated position. Hence the contact RLCl serves to alter the voltage on the output winding 10 and it will be seen that the arrangement of Figure 2 thus operates in exactly the same Way as the circuit of Figure 1. However, the circuit of Figure 2 facilitates adjustment of the output voltage since the two resistors 31, 32 may be connected to a single adjustable tap 30 on the winding 12. The output voltage on winding 10 may be then adjusted as required by altering the tap 30.

I claim:

1. A circuit for protecting equipment embodying an electronic valve in the event of an excessive current being drawn by the valve, which circuit comprises a supply source for supplying two alternative voltages one higher than the other, relay-operated switch means for applying the two alternative voltages to said equipment in two stages the lower voltage being applied before the higher voltage, and means responsive to the current drawn from the supply source arranged immediately to interrupt the high voltage supply circuit in the event of an excessively large current being drawn from said supply source and subsequently to initiate operation of said switch means for re-applying the high voltage in two stages.

2. A circuit for protecting equipment embodying an electronic valve in the event of an excessive current being drawn by the valve, which circuit comprises an alternating voltage supply, a transformer having an input winding and an output winding at least one of which is adjustably tapped, circuit means connecting said input winding to said supply and said output winding to said equipment, relay-operated switch means for switching connections to said tapped winding whereby two alternative voltages are applied to said equipment, the lower voltage being applied before the higher voltage, and means responsive to the current drawn from the supply source arranged immediately to interrupt the high voltage supply circuit in the event of an excessively large current being drawn from said supply source and subsequently to initiate operation of said switch means for re-applying the high voltage in two stages.

'3. A circuit for protecting equipment embodying an electronic valve in the event of an excessive current being drawn by the valve, which circuit comprises an alternating voltage supply, a transformer having an input winding and an output winding, 21 first circuit connecting said input winding to said supply, a second circuit connecting said output to said equipment, an impedance, relay-operated switch means for switching said impedance into or out of one of said first and second circuits whereby two alternative voltages are applied to said equipment, the lower voltage being applied before the higher voltage, and means responsive to the current drawn from the supply source arranged immeidately to interrupt the high voltage supply circuit in the event of an excessively large current being drawn from said supply source and subsequently to intiate operation of said switch means for re-applying the high voltage in two stages.

4. A circuit for protecting equipment embodying an electronic valve in the event of an excessive current being drawn by the valve, which circuit comprises a supply source for supplying two alternative voltages one higher than the other, a voltage control relay, a circuit including contacts of said voltage control relay for applying the two alternative voltages to said equipment in two stages, said voltage control relay being arranged normally to apply the lower voltage to the equipment and to apply the higher voltage only after the lower voltage has been applied for a delay period, and means responsive to the current drawn from said supply source arranged immediately to interrupt the high voltage supply circuit in the event of an excessively large current being drawn from said source and subsequently to initiate operation of said voltage control relay for re-applying the voltage to said equipment in two stages.

5. A circuit for protecting equipment embodying an electronic valve in the event of an excessive current being drawn by the valve, which circuit comprises a supply source voltage-control relay, an impedance, a circuit for applying the voltage from said source to said equipment into and out of which circuit said impedance can be switched by said voltage-control relay whereby two alternative voltages may be applied to said equipment one higher than the other, said voltage-control relay being arranged normally to apply the lower voltage to the equipment and to apply the higher voltage only after the lower voltage has been applied for a delay period, and means responsive to the current drawn from said supply source arranged immediately to interrupt the high voltage supply circuit in the event of an excessively large current being drawn from said source and subsequently to initiate operation of said voltage control relay for re-applying the voltage to said equipment in two stages.

6. A circuit for protecting equipment embodying an electronic valve in the event of an excessive current being drawn by the valve, which circuit comprises an alternating voltage supply, a transformer having an input winding and an output winding at least one of which is adjustably tapped, a voltage-control relay, circuit means connecting said input winding to said supply and said output winding to said equipment, which circuit is arranged to be switchably connected to the tapped winding by said voltagecontrol relay so that two alternative voltages may be applied to said equipment one higher than the other, said voltage-control relay being arranged normally to apply the lower voltage to the equipment and to apply the higher voltage only after the lower voltage has been applied for a delay period, and means responsive to the current drawn from said supply source arranged immediately to interrupt the high voltage supply circuit in the event of an excessively large current being drawn from said source and subsequently to initiate operation of said voltage control relay for re-applying the voltage to said equipment in two stages.

7. A circuit for protecting equipment embodying an electronic valve in the event of an excessive current being drawn by the valve, which circuit comprises a supply source, relay-operated switchable circuit means for deriving two alternative output voltages from said source one voltage being higher than the other, means for applying the output voltage from said switchable circuit means to said equipment, a delay control for said switchable circuit means arranged to control the circuit means so that the higher voltage is applied to the equipment only after the lower voltage has been applied for a delay period and means responsive to the current drawn from the supply source arranged immediately to interrupt the supply to said switchable circuit means in the event of an excessively large current being drawn from said supply source and subsequently to initiate operation of said relay-operated switchable circuit means for re-applying said output voltage to said equipment in two stages.

8. A circuit for protecting equipment embodying an electronic valve in the event of an excessive current being drawn by the valve, which circuit comprises a supply source, relay-operated switchable circuit means for deriving two alternative output voltages from an applied supply one being higher than the other, means for applying the output voltage from said switchable circuit means to said equipment, a delay control for said switchable circuit means arranged to control the circuit means so that the higher voltage is applied only after the lower voltage has been applied for a delay period, an overload relay having an operating coil controlling a first normallyclosed pair of contacts and a second normally-open pair of contacts, circuit means for energising the operating coil in accordance with the-current fed to said equipment through said switchable circuit means so that the overload relay operates only when an excessive current is fed to said equipment, a delay relay having normally-closed pair of contacts, circuit means including said second normally open pair of contacts of said overload relay arranged to operate said delay relay when said overload relay operates, a release circuit for said delay relay arranged to release said delay relay automatically at a pre-deterniincd time after it has been operated, a connecting circuit including said first normally-closed pair of contacts of said overload relay in shunt with said normally-closed pair of contacts of said delay relay connecting said supply source to said switchable circuit means for supplying power thereto, and a control circuit operably controlled by said delay relay to condition said switchable circuit means to supply the lower voltage to said equipment when said delay relay is operated.

References Cited in the fileof this patent UNITED STATES PATENTS 

